1. Field of the Invention
The present invention relates to composite dielectric filters formed by arranging conductive films inside and outside dielectric blocks, and communication apparatuses incorporating the composite dielectric filters.
2. Description of the Related Art
The above-mentioned filters are used as band pass filters and the like in a microwave band. Particularly, in a single dielectric block, there are provided a duplexer formed by arranging a transmission filter passing signals through a transmission frequency band and inhibiting signals from passing through a reception frequency band and a reception filter passing signals through the reception frequency band and inhibiting signals from passing through the transmission frequency band. The duplexer is used as an antenna duplexer incorporated in an apparatus such as a mobile phone.
In terms of a composite dielectric filter device using a single dielectric block including such a plurality of filters, one important point in designing the device is to secure the isolation between the filters. For example, the duplexer as the antenna duplexer is used to isolate transmission signals and reception signals. On the other hand, when the transmission signals are sent to a reception circuit, this has negative effects on the reception signals and thereby reception characteristics are deteriorated. As a result, the antenna duplexer can obtain characteristics capable of significantly attenuating transmission signals in the reception frequency band.
However, with the current trend toward the miniaturization of communication apparatuses, more compact composite dielectric filter devices have been manufactured. Consequently, there is a problem in that it is difficult to obtain isolation characteristics for measuring up to desired values.
Accordingly, it is an object of the present invention to provide a composite dielectric filter device capable of easily improving the isolation characteristics between adjacent filters even when using a compact dielectric block to miniaturize the entire configuration of the device. It is another object of the invention to provide a communication apparatus incorporating the composite dielectric filter device.
According to a first aspect of the invention, there is provided a composite dielectric filter device including a parallelepiped rectangular dielectric block, a plurality of inner conductors extending in parallel from one face of the dielectric block to the opposite face, an outer conductor arranged on at least some of the outer faces of the dielectric block so that groups of adjacent inner conductors constitutes a plurality of filters, and a outer-conductor-free portion formed at a part of the outer conductor corresponding to the boundary between the mutually adjacent filters. With this arrangement, the coupling between the ground currents of the adjacent filters, that is, the inductive coupling between the ground current of one of the filters and the ground current of the remaining filter can be suppressed. As a result, the isolation characteristics between the mutually adjacent filters can be improved.
In addition, the outer-conductor-free portion may be formed around all the outer faces of the dielectric block. With this arrangement, since the coupling between the ground currents of the filters can be suppresses without fail, the isolation characteristics between the adjacent filters can be improved.
In addition, the composite dielectric filter device may further include input/output terminals extending from one of the outer faces of the dielectric block to another face thereof. The terminals may be isolated from the outer conductor and shared by the adjacent two filters and the outer-conductor-free portion. The outer-conductor-free portion may be continuous with the periphery of the terminal. With the outer-conductor-free portion continuous with the periphery of the input/output terminal, the effect of suppressing the coupling between the ground currents of the adjacent filters can be enhanced.
Furthermore, the composite dielectric filter device may further include ground-connected metal covers connected to the outer conductors of the dielectric block. The metal cover may be arranged independently for each of the outer conductors formed by separating at the outer-conductor-free portion. With the ground-connected metal cover independently arranged for each filter, the coupling between the ground currents of the adjacent filters can be suppressed.
According to a second aspect of the invention, there is provided a communication apparatus incorporating the composite dielectric filter of the invention. The filter is used as an antenna duplexer. This arrangement can sufficiently prevent a transmission signal from being input to a reception circuit and therefore satisfactory reception characteristics can be obtained.